1. Field of the Invention
This invention relates generally to a method and apparatus for packaging articles in a heat shrinkable film and in particular to a method and apparatus for controlling the volume and shape of the shrinkable film as it passes through a heat shrink tunnel without the film prematurely touching the article.
2. Description of Related Art
The use of plastic films known as shrink film for packaging food or other products is an art that dates back to the 1930's. It is well established that when such heat shrinkable films are exposed to a blast of hot air or bath of hot water they will contract to a much smaller film dimension. These films are generally specially treated polyolefins, irradiated polyolefins, polyolefin copolymers, polyvinyl chloride or polyesters.
The first thing that occurs in the shrinking process is that the film is drawn around an article and sealed. This makes an air tight (with the exception of small vent holes) film bag (bubble). In order to shrink the film in an even manner, without excessive wrinkles and/or film bunching on package corners (known as "ears"), various devices were devised to vent air from the film bag. These holes in the film allowed the entrapped air to escape as the film shrinks under the influence of heated air. Such holes are known in the art as "vent holes".
Prior art focused on controlling the rate of speed of this air exhaust. Vent holes were originally die-cut approximately 1/4" in diameter or burned (size was more random) in the film. Typically two vent holes would be provided to exhaust the air. Later the hole was decreased to 3/16" or 1/8" diameter to improve venting by delaying the total collapse of the film bubble around the package as long as possible to effect a better looking package. A still further improvement of controlling air exhaust rate, was found by providing a series of small pin holes in place of a single hole. In each case the progression of improvements resulted in a package with less wrinkling and smaller ears.
The problem with all of these prior art improvements is that the articles with very thin or flat side walls, very high side walls, such as a cube, or an article with high thermal conductivity, for example metal spray cans, or an article with an irregular shape, leaves most packages with unacceptable wrinkling and ears, unless considerable time is spent in adjusting air flow, belt speed, etc., in the shrink tunnel. In fact with any article, considerable time is spent adjusting wrappers and tunnels to improve package appearance.
In the ideal situation, when a thin film shrinks onto a product there would be no interaction between the product and the film until the film is completely shrunk around the package during the shrinking process. The product, by not coming in contact with the film, would be unable to remove heat from the film as the film contracts during the shrinking process. As a result the film would be able to evenly shrink around the product without having heat sucked out as it touches, for example, the side or end walls of the product.
In the extreme, a product can have a high thermal mass such as a frozen turkey. This mass can have a tremendous effect on the quality of the shrink. In the case of the frozen turkey, the only way to get a smooth wrinkle free shrink is to use a water bath just below boiling temperature. If shrink wrapping is carried out on a frozen turkey in a hot air tunnel there would be very poor shrinking with many wrinkles. This is due to the thermal mass of the turkey being far greater than the driving force of the hot air. The frozen turkey pulls the heat out of the film faster than the hot air can heat the film.
U.S. Pat. No. 5,009,057 issued Apr. 23, 1991 to Frank G. Wilkinson discloses a method and apparatus for shrink wrapping comprising a hot air chamber for blowing hot air on the shrink film while an article covered by the shrink film is on a conveyor and a plurality of rollers mounted on a support frame above the conveyor. This method requires holding the shrink film covered article down against the tunnel conveyor by exerting pressure from at least one of the plurality of rollers on the covered article. This method is intended for use with light objects which float or thin or flimsy articles that are easily deformed or warped. Wilkinson teaches the use of silicon rubber covered roller cylinders to transmit heat to an article. The high thermal conductivity of these rollers can actually melt the film during shrinking.
U.S. Pat. No. 5,339,605 issued Aug. 23, 1994 to Billy J. Simpson et al. discloses a shrink tunnel having an upper secondary conveyor adapted to compress a wrapped article during an initial period of the article's passage through the shrink tunnel removing any air trapped between the folds of a textile article such as bed sheets. However, this apparatus requires driven rollers and the additional overhead conveyor for compressing the article. This method is also intended for use with light objects which float or thin and flimsy articles that are easily deformed or warped. This is expensive and time consuming to install into an existing shrink tunnel.
U.S. Pat. No. 5,400,570 issued Mar. 28, 1995 to Charles J. Bennet describes a method for heat shrinking film around a cold food product. In shrink tunnels using hot air, shrinking stops when the film contacts cold food even in the presence of hot moving air. Bennet describes the use of a combination of a first assembly for providing heated air to a bottom portion of the film and a second assembly for directing heated water into at least a top portion of the film envelope. However, this invention requires complex equipment in order to overcome the problems related to shrinking films on cold products.